340-4spDuster, thanks for the chart. Just got my '68 340 running and my plugs are about the OK to the left of Best. Running a 10.25:1 CR Sealed Power pistons with a 272° cam, RPM manifold and TQ carb. 12° initial advance and 34° total with vacuum advance operational. Timing line is about perfect right above the plug base. Just a hint of carbon deposits inside the plug so it is running a little rich. Not bad for stock jetting running at 5800'. Will play with metering rods to get the A/F ratio spot on.
I see no need for HEI ignition if your stock coil and wires are in good condition. I run an electronic ignition from the '70's with no issues. The engine will idle at 500 RPM or even lower but oil pressure starts falling at that low of RPM. Usually, it is 40 PSI at 750 but drops to 30 PSI at 500.
I would get a Leak Down tester from Harbor Freight or build one yourself.
Build you own leak down tester - Search Videos and many other such sites.
This is valuable for troubleshooting many engine build issues including leaking valves, gaskets, rings, etc. Much more info than a compression test, but both are worthwhile and give different info.